1. Field of the Invention
The present invention relates to an air flow measuring apparatus for measuring air flow sucked from an air intake pipe by converting it into electrical signals so as to electronically control a fuel injection unit for an internal combustion engine.
2. Description of the Prior Art
A measuring plate type air flow measuring apparatus is widely known in which an intake air flow is measured by converting it into electrical signals, e.g. a voltage output by a potentiometer arranged in the intake pipe of the internal combustion engine, which potentiometer interlocks with a measuring plate which is pivotally displaced in accordance with the intake air flow.
However, such an air flow measuring apparatus as described above was developed as an analogue control type electronic fuel injection control device and has the following problems when used for an electronic fuel injection control unit of a digital control type in which highly accurate control is possible by using a microprocessor (which has rapidly come into wide use).
Namely, in the electronic fuel injection control unit of the digital control type, an air flow measuring range (the ratio between the minimum air flow and the maximum air flow) necessary for an intake air flow measuring apparatus is generally about 1:40 and measuring errors of air flow must be maintained less than .+-.2% regardless of any air flow zone in view of the power of the internal combustion engine. Purification is also measured with respect to exhaust gases. For these reasons, the measuring accuracy must be maintained within: 1/40.times.2/100=1/2000.apprxeq.1/2048=1/2", so that the voltage output of the air flow thus measured in the digital control type electronic fuel injection control apparatus is converted into digital signals by an analog/digital converter (A/D converter) with more than 11 bits.
However, mass production elements commercially available such as microprocessors and A/D converters are all 8-bit systems. Accordingly, if these 8-bit systems can be used as electronic fuel injection control systems (ECU), not only can the systems be extremely simplified in hardware and software aspects, but also the operating efficiency necessary for air flow measuring operation in the central processing unit (CPU) for the microprocessor can be bettered when processing equal volumes of data.
Furthermore, they also enable the ignition period of an ignitor as a subsystem to be controlled, thus enabling the cost of the electronic injection control apparatus to be lowered.
However, as described in the foregoing, as a more than 11-bit A/D converter is required in the conventional air flow measuring apparatus, there were drawbacks preventing simplification of the systems as well as preventing large scale cost-reductions.
Moreover, improvement in fuel consumption for especially motor vehicles has been socially demanded. As one of the effective means for realizing this, there is an idle speed control method (ISC) which tries to restrict the engine speed so that minimum air flow will become small.
On the other hand, there is a tendency that the maximum air flow increases by using a double overhead cam mechanism, (DOHC) or by performing surcharging through a turbo-charger. As a result, a wide dynamic range such as 1:70 to 1:80 in the measuring range of a vacant air flow measuring apparatus has been demanded, and this in turn necessitates an A/D converter with large bit numbers in order to accurately measure the air flow by the prior art air flow measuring apparatus, thus making the cost of ECU to be more and more high.